The demand for increased bandwidth in fiberoptic telecommunications has driven the development of sophisticated transmitter lasers usable for dense wavelength division multiplexing (DWDM) systems wherein multiple separate data streams propagate concurrently in a single optical fiber. Each data stream is created by the modulated output of a semiconductor laser at a specific channel frequency or wavelength, and the multiple modulated outputs are combined onto the single fiber. The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz, which allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Greater bandwidth requirements will likely result in smaller channel separation in the future.
Telecom DWDM systems have largely been based on distributed feedback (DFB) lasers. DFB lasers are stabilized by a wavelength selective grating that is pre-determined at an early step of manufacture. Unfortunately, statistical variation associated with the manufacture of individual DFB lasers results in a distribution of (wavelength) channel centers. Hence, to meet the demands for operation on the fixed grid of telecom wavelengths (the ITU grid), DFBs have been augmented by external reference etalons and require feedback control loops. Variations in DFB operating temperature permit a range of operating wavelengths enabling servo control; however, conflicting demands for high optical power, long lifetime, and low electrical power dissipation have prevented use in applications that require more than a single channel or a small number of adjacent channels.
Continuously tunable external cavity lasers have been developed to overcome the limitations of individual DFB devices. Many laser tuning mechanisms have been developed to provide external cavity wavelength selection, such as mechanically tuned gratings used in transmission and reflection. External cavity laser tuning must be able to provide a stable, single mode output at a selected wavelength while effectively suppress lasing associated with external cavity modes that are within the gain bandwidth of the cavity. Achieving these goals typically has resulted in increased, size, cost, complexity and sensitivity in tunable external cavity lasers.
There is accordingly a need for an external cavity laser and tuning mechanism therefore that prevents multimode lasing by effective suppression of transmission peaks at wavelengths other than a selected wavelength, that is simple and compact in design, and which is straightforward to implement. The present invention satisfies these needs, as well as others, and overcomes the deficiencies found in the background art.